There are many situations in which it is desirable or even necessary to determine the level of moisture in an atmosphere. For instance, in testing the permeability of a material to water vapor. One such instance is the testing of the water permeability of a plastic film or sheet used to form a package for a product. A test instrument is provided which passes a first water vapor containing atmosphere across one side of the plastic film or sheet and a second water vapor free atmosphere across the opposite side of the plastic film or sheet. Measurement of the water vapor level in the second atmosphere, after it has passed over the opposite side of the plastic film or sheet, provides an indication of the water vapor permeability of the plastic film or sheet. In such applications, the water vapor level in the second atmosphere will start out very low and increase as water vapor from the first atmosphere permeates the plastic film or sheet. Thus, it would be desirable to initially accurately read low levels of water vapor in the second atmosphere, and as more water vapor permeates the plastic film or sheet, thereafter read higher levels of water vapor in the second atmosphere. However, in other applications it is desirable to first measure high levels of water vapor, and thereafter measure lower levels. Such as, when it is desirable to monitor moisture being removed from an atmosphere and it is necessary to indicate when the moisture vapor level has reached a desired low level.
Separate systems and sensors have long been available to measure either high levels of water vapor or low levels of water vapor. However, a single testing apparatus and system has not been provided which will respond to water vapor levels in an atmosphere to accurately indicate both high levels of moisture vapor and low levels of moisture vapor, without the need to determine in advance whether high or low levels of water vapor are present in the atmosphere being observed. It has been necessary in the past to have prior knowledge of the water vapor level, since exposure of a typical low level water vapor sensor to other than a very short exposure to high moisture vapor levels would render it either inoperative, until it is dried, or even possibly destroy the sensor.
Accordingly, it would be of considerable advantage to provide a system and method in which a single instrument is provided to indicate both very low levels and also high levels of water vapor in an atmosphere without the need to first determine the water vapor level, so as not to expose a sensor for low levels of water vapor to a high level of water vapor, which would either destroy the sensor or render it inoperative until it is dried for a considerable period of time.